linux-kernel - Re: [PATCH 00/14] Fix wrong %pF and %pS printk format specifier usages

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Date:   Fri, 8 Sep 2017 20:28:13 +0200
From:   Helge Deller <deller@....de>
To:     "Luck, Tony" <tony.luck@...el.com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH 00/14] Fix wrong %pF and %pS printk format specifier
 usages

On 08.09.2017 19:25, Luck, Tony wrote:
> On Fri, Sep 08, 2017 at 03:18:30PM +0900, Sergey Senozhatsky wrote:
>> if the addr is not in kernel .text, then try dereferencing it and check
>> if the dereferenced addr is in kernel .text.
> 
> If it really is a function pointer, then we know that it is safe
> to dereference. But if it isn't, then maybe not?
> 
> If it is a function pointer then dereferening will indeed give
> us a .text address. But if it isn't, it might still give us a
> .text address (we could reduce the probability of a false hit
> by checking that the .text address was exactly on a symbol with
> no offset ... but data values that happen to be the addresses of
> function entry points are possible).

I don't like this kind of trying to figure out at runtime at all.
It's too much guessing in here IMHO.

What about this idea:
For %pF we always have pointers to functions, e.g.: 
        printk("Going to call: %pF\n", gettimeofday);
        printk("Going to call: %pF\n", p->func);

and for %pS most (if not all) usages use some kind of casting 
from "unsigned long" to "void *", e.g.:
        printk("%s: called from %pS\n", __func__, (void *)_RET_IP_);
        printk("%s: called from %pS\n", __func__, (void *)__builtin_return_address(0));
        printk("Faulted at %pS\n", (void *)regs->ip);

So, what if we for the %pS case simply take the type as it is 
(unsigned long) and introduce a new printk-format, e.g. "%luS" ?
The %pS examples above then become:
        printk("%s: called from %luS\n", __func__, _RET_IP_);
        printk("%s: called from %luS\n", __func__, __builtin_return_address(0));
        printk("Faulted at %luS\n", regs->ip);

That way we don't need type-casting, gain compile-time type 
checks from the compiler, and we could add a checkpatch (or occinelle)
check which checks for the combination of %pF/%pS and "void*" keyword
and suggest to use %luS.

Opinions?

Helge